In flexographic printing, there is provided a print roll having raised print surfaces. This is accomplished by providing a main support roll (i.e. a plate cylinder) having a generally cylindrical surface on which a printing plate with raised letters or characters is secured. The printing plate is usually formed by a supporting or backing plate which is generally of small thickness such as about 0.030 inch, and one or more raised print plate parts are secured to and project upwardly from the backing, with the print plate parts having a thickness which varies but typically are about 0.250 inch. Typically a liquid ink is used which is applied from the source, such as a typical ink supply fountain using a doctor blade, to an intermediate roller which is normally referred to as an anilox roll. This anilox roll has finely engraved meshlike depressions in its surface for picking up the ink. A doctor blade or other means remove excess ink from the anilox roll surface. The anilox roll then transfers the ink to the raised print areas on the print roll, and the latter in turn effects printing on the work product such as a corrugated paperboard, paper or the like.
During printing, particularly after a significant number of printing cycles, debris tends to collect on the print plate which hence interferes with precise printing. This debris typically is minute paper particles which are associated with the paperboard being printed. Due to this collection of debris on the print roll, or due to the necessity of having to change ink colors, it is necessary to periodically clean or wash the print plate. In the past, the washing of the print plate on flexographic printers has typically been a manual operation, requiring separation of printing sections of the machine and loss of print color registration and has occasionally required removal of the plate from the cylinder in as much as manual washing of the print plate on the machine is messy and complex.
While attempts have been made to provide a washing apparatus for a rotary print roll, these attempts have been primarily in relationship to lithographic (i.e. offset) printers, and the washing arrangements which have been utilized for lithographic printers are not believed to possess all of the properties necessary or desirable for use in effecting proper washing of a printing plate on a flexographic printer.
More specifically, in lithographic printing the print roll is not provided with raised print areas. Rather, a lithographic print roll typically has a rubber blanket which surrounds the roll and defines thereon a flat or generally planar printing area, in contrast to raised print areas in flexographic printing. The printing blanket on the lithographic roll receives ink from an imaged printing plate or transfer roll, and the overall process for picking up and transferring ink involves the use of hydrophilic and hydrophobic areas on the printing plate for controlling the ink and affecting printing of the desired material. Because of the smooth or generally flat print surface defined on the blanket of the lithographic print roll, cleaning of the roll is a much simpler process because of the lack of surface variations and protrusions which exist on a flexographic print roll. Further, the smooth surface characteristic of the lithographic print roll often normally permits the cleaning to be accomplished during the normal rotational cycle of the lithographic print roll, and such is generally not feasible with a flexographic print roll.
Examples of prior art attempt to provide cleaning of print rolls, including lithographic print rolls, are shown in the following U.S. Pat. Nos.: 4,270,450, 4,393,778, 5,010,819, 5,209,164, 5,235,913, 5,265,537 and 5,303,652.
Accordingly, it is an object of this invention to provide an improved washing arrangement for cooperation with a rotary print roll to effect cleaning of ink and debris from printing areas on the print roll, which washing apparatus is particularly suitable for use with print rolls having raised print areas such as a flexographic print roll.
More specifically, in the present invention a washing apparatus is positioned adjacent the periphery of the print roll and extends longitudinally along the length thereof. The washing apparatus includes an elongate housing which mounts thereon a first longitudinally elongate spray bar which mounts one or more nozzles for directing a cleaning liquid against the peripheral surface of the print roll. The housing also mounts a second spray bar which includes one or more nozzles for directing a rinsing liquid against the peripheral surface of the print roll, with the rinsing spray bar being spaced from the first-mentioned spray bar in the direction of rotation of the print roll. A moving brush is also positioned in the housing generally between the spray bars for effecting brushing of the raised print areas after the cleaning liquid has been applied thereto. The liquid and debris used in the cleaning operation collect in the housing and are evacuated therefrom.
In the improved washing arrangement of the invention, as aforesaid, the housing is preferably formed as an elongate channellike trough open on one side which generally faces the periphery of the print roll. The spray bar nozzles extend longitudinally of the trough and are directed to discharge outwardly through the mouth of the trough for applying the cleaning and rinsing liquids to the print areas. The trough is generally oriented with the mouth thereof projecting somewhat sidewardly toward the roll so that the cleaning nozzles are positioned above the rinsing nozzles to facilitate the flow and collection of the liquids and debris.
In the washing apparatus of the invention, as aforesaid, the trough is provided with sealing strips formed preferably as brush strips which extend along both the longitudinally and end edges of the trough opening for maintaining a rubbing engagement with the print roll. These edge brushes maintain contact with the basic surface of the print cylinder in those areas not contacted by the backing of the printing plate, with the brushes being suitably deflectable so as to also maintain proper engagement with both the backing or the print areas as the print roll moves past the washing apparatus. The edge brushes act as partial seals so as to maintain the liquids and debris within the cleaning region defined between the open trough and the periphery of the print roll.
In the washing arrangement of this invention, as aforesaid, the housing or trough is preferably mounted on a pivot arrangement which enables it to pivotably move between an operational position adjacent the print roll and a non-operational position pivoted outwardly away from the print roll when washing is not required. Pivot supports are preferably provided at each end of the housing or trough. The pivot shaft at one end also preferably is hollow at least in part, and extends generally into the lower portion of the collection cavity defined within the trough, and an outer end of the pivot shaft is connected to a suitable vacuum source so as to effect evacuation of the liquids and debris which collect within the trough during the washing operation. The edge brushes also are effective in permitting limited quantities of outside air to flow through the brushes into the interior of the trough so as to effect confinement of the washing and rinsing liquids within the trough and evacuation thereof through the suction tube.